Once the Shipstation integration is active and setup in your account, you will need to set what Shipstation products your Pressero products will be associated with. If you have Shipstation setup and are not getting any orders showing in Shipstation, that can indicate that the products on the orders are not associated to any products in Shipstation.
You need to edit any products on your site and associate them to the Shipstation product that will apply to those orders. Edit the product on the site. Go to the Embedded Services tab for that product, to the Options > Product dropdown list setting, and select the Shipstation product to be associated to that product in Pressero.
If you have a lot of products to associate to a Shipstation product, you can also consider using the default setting that will apply the Shipstation product automatically to any orders. Go to Preferences > Embedded Services in your account, edit the Shipstation item, and then you can use the Default Product SKU field to assign a default value for the Shipstation product to use for any orders.